Go for it.. It NEVER hurts to have just a little more speaker here and there...
Ok, from the looks of the photo, your rear '6th' speaker would be in a corner ? Directly across from the corner the T/V is sitting in ? If so, a QS8 would be ideal.. Sending out all it's marvalous little sounds, bouncing off the two angled walls, what could be better ? You wouldn't even need a pair, one would do the trick..

LFE ! The rest is just details..